Creative Commons - Make sure to uncheck both boxes ("use for commercial purposes" and "modify, adapt, or build upon"). You can use Creative Commons to search for copyright-free images, videos, and music from sources like Google Images, Flickr, and Open Clipart Library.
Digital Public Library of America - The DPLA allows free access to digital materials from America's libraries, archives, and art museums.
Pics4Learning - Pics4Learning is a copyright-friendly image library for teachers and students. The Pics4Learning collection consists of thousands of images that have been donated by students, teachers, and amateur photographers. Unlike many Internet sites, permission has been granted to teachers and students to use all of the images donated to the Pics4Learning collection.
*Remember, you still must cite the images you use in your project!